The considerable development of mobile telephony with its latest enhancements requires the use of increasingly sophisticated tunable filters.
Considering in particular the Wideband Code Division Multiple Access (WCDMA) standard for the 3rd generation mobile telephony, based on 2.14 Ghz, it can be considered to realize bandpass-type filter circuits requiring the use of resonator circuits. But making such circuits using passive devices of the LC-type is particularly delicate and it is even more difficult to integrate such circuits into a semiconductor product.
Using SAW resonators is known, unfortunately, such resonators do not allow embedding in an integrated circuit.
It has been sought to integrate a BAW-type acoustic resonator in an integrated circuit, but manufacturing of such resonators is particularly difficult to control. French Patent Application No. 03 15480, filed on 29th Dec. 2003, describes how to realize a filter element comprising a BAW resonator.
However, combining independent filter elements in order to make a sophisticated filter circuit remains a delicate operation since it comprises combining a great number of components, in particular of inductive resistors, varactors, and BAW resonators.
In particular, all components, and more particularly inductive resistors, should have a high coefficient of quality. Moreover, it is important to make sure that inductive resistors are not mutually disrupting.
Generally, it would be desirable to reduce as much as possible the occupied surface in an integrated circuit. It is indeed current that a planar inductive resistor on a substrate of silicon occupies a surface about 400×400 micrometers. It is clear that a great number of inductive resistors is then crippling to integrated circuit manufacturing.
Indeed, it is pointed out that the typical size of a BAW resonator is about 150×150 micrometers.